Dust, fume, and odour: why knowledge is power

As with all dust, fume and odour control applications for waste processing, the first stage is to understand what generates the dust, fume and odour. Then a solution can be put in place, say Adrian Sharpe and Andy Quail

When looking at waste handling and recycling, the odour-generating process starts in the dustbin. Once deposited in the bin, the decaying/putrefaction process begins. Anaerobic putrefaction begins due to a lack of oxygen, resulting in the production of sulphide compounds, amines and mercaptans.

Once this process has started, the action of emptying bins, transporting the waste and processing it at the transfer stations and subsequent process plant releases the odours to the atmosphere along with the dust contained within the waste. Having assessed what causes the problem, the following points must be considered when selecting equipment for a suitable abatement plant.

The nature and materials to be collected should be considered as, for example, some of the particulate matter will be wet and will have already started to putrefy. Due to putrefaction and decay, both acid and alkaline odorous gases will be present that will need to be removed. Bacteria that cause the anaerobic putrefaction and decay will also be present.

Having this information allows certain types of abatement plant to be ruled out. Barrier filters, for example, will not be able to handle the wet sticky particles of putrefying material and will quickly become blocked. The collected material or 'cake' formed on the filter elements during the filtering process would also become a food source for any bacteria present.

Pros and cons
While carbon filters can take care of the odours, these filters would quickly become blocked due to the nature of the particulate. The carbon would also be food for bacteria. While misting systems would reduce the larger particles in the air, they miss fine particles and cannot address the odour problem. There are also health and safety risks as well as problems associated with vehicles and personnel working on damp and slippery floors, compounded by the inevitable risk of spillage.

One abatement plant solution that Carter Environmental Engineers offers is the Cyclovent with a second-stage gas scrubber, which gives minimal emission levels. The Cyclovent multi-throat venturi scrubber uses cyclonic and high-pressure venturi technologies which gives a number of advantages.

It reduces operating pressure compared to conventional venturi type scrubbers and has high solids handling capabilities and low maintenance requirements. It also has minimum space requirements and low pumping power requirements.

Having removed the particulate and some of the odorous gases in the Cyclovent, a second stage is required to remove the remaining fumes and odours. This can be achieved using a packed gas scrubber with the appropriate liquor. The gas scrubber incorporates a unique 'V' trough liquor distribution system to ensure that the gases achieve maximum contact with the scrubbing liquor.

Using the above system with appropriate pH control and oxidising reagent dosings, typical plant efficiencies that can be achieved are as follows: > 25 micron, 99.99%; <25 > 10 micron, 99.9%; <10 > 5 micron, 99.85%; <5 >2 micron, 99.8%; <2 >1 micron, 98.5%; <1 > 0.5 micron, 92%; <0.5 >0.2 micron, 70%. These readings are based on particles with aerodynamic size and specific gravity of 2.2.

Find the source first
Odour removal depends on the source of the odour. Odours generated by bacteria will be greatly reduced by the strong oxidising reagents maintaining a redox potential at 600 ensuring that there is surplus oxidising agent.

If the odour is due to inorganic chemicals, a reduction factor of 1096 will be achieved; with a concentration of 140ppm entering the abatement plant the concentration will be 0.125ppm exiting. This is a slight simplification as certain substances react more than others. Carbon monoxide reduction will be trivial whereas sulphur dioxide reduction will be greater than 1000-fold.

Waste handling relies on waste being collected and transported either to a transfer station or direct to waste handling or incineration plants. As soon as the waste is delivered to the waste station the release of dust, fumes and odour is inevitable as the waste stream is processed. If the dust, fume and odour are extracted at source, scrubber site emissions can be kept to an absolute minimum and well within legislative requirements.

With an appropriately designed extraction system, dust, fumes and odours can be captured at source, improving efficiency and keeping costs to a minimum.
>Adrian Sharpe is senior sales/projects engineer and Andy Quail is sales & commercial director at Carter Environmental Engineers



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